Boron Nitride vs Graphite: Which High-Temperature Lubricant To Use, And When
Hexagonal boron nitride and graphite are both layered solid lubricants, which is why h-BN is called white graphite. The differences decide the application. Graphite is black, electrically conductive and starts to oxidise away in air at roughly 450–500 °C; h-BN is white, electrically insulating, and holds up in air to about 850–900 °C, in vacuum to about 1400 °C and in inert gas higher still. Graphite is cheaper per kilogram and is the right choice for hot forging of steel, where die temperatures stay within its limit and a black film does not matter. Boron nitride is the right choice wherever the part must come out clean (aluminium extrusion, die casting, glass pressing, brazing), wherever the process runs in vacuum or inert gas, or wherever carbon pickup or electrical conduction would be a defect. Pyroflux India makes both: graphite-based and graphite-free forging lubricants, and the Pyrolube range of h-BN sprays and suspensions.
The Properties That Decide It
| Property | Graphite | Hexagonal boron nitride |
|---|---|---|
| Colour of the film | Black | White |
| Oxidation limit in air | Begins at roughly 450–500 °C | Roughly 850–900 °C |
| In vacuum or inert gas | Lubricity falls off in vacuum (it needs adsorbed moisture to shear well) | Keeps lubricating; usable to about 1400 °C in vacuum, higher in inert gas |
| Electrical behaviour | Conductive | Insulating (volume resistivity above 10¹³ Ω·cm) |
| Thermal conductivity | High in-plane | High in-plane; the thermal-filler material of choice where insulation is also needed |
| Wetting by molten aluminium | Partially wetted; reacts over time | Not wetted |
| Residue on the part | Carbon pickup, black marks | None; inert white trace at most |
| Hardness (Mohs) | 1–2 | About 2–2.5 |
| Cost per kilogram | Low | Several times higher |
| Pyroflux products | Forgecoat 100G, graphite mould coatings, Aeroflex gasket coat | Pyrolube 20, 25S, BN 5026, BN aerosol, BN wax sticks |
When To Choose Which
| Process | Use | Why | Pyroflux grade |
|---|---|---|---|
| Hot forging of steel | Graphite | Die temperatures within graphite’s range; black film acceptable; cheapest per part | Forgecoat 100G; 100S where a clean film is required |
| Aluminium extrusion (billet, dummy block, die) | Boron nitride | No carbon marks on profiles, no wetting by aluminium, no acetylene | Pyrolube 20, BN wax sticks |
| Pressure die casting (cores, soldering areas) | Boron nitride | Non-wetting, white, no residue on castings that will be painted | BN aerosol, BN suspension |
| Casthouse launders, ladles, crucibles | Boron nitride | Barrier to molten aluminium; graphite would be attacked | Pyrolube BN 5026 |
| Glass pressing | Boron nitride | Glass does not wet BN; graphite smokes and marks the ware | Pyrolube glass system |
| Brazing fixtures, sintering setters | Boron nitride | Must not contaminate the joint or the part; often inert or vacuum atmosphere | BN aerosol or suspension |
| Furnace hardware in air above 500 °C | Boron nitride | Graphite burns away | BN suspension or paste |
| Gasket sheet top coat | Graphite (or BN for white) | Room-temperature application, familiar grey face, cost | Aeroflex |
| Foundry mould and core coating (iron, steel) | Graphite, zircon, alumina-silicate | Refractory barrier against ferrous metal; BN is not the economical choice here | Mould coatings |
A Rule Of Thumb
If the part is steel and the film can be black, use graphite and save money. If the part is aluminium, magnesium, zinc or glass, or if the film must be white, insulating or carbon-free, or if the process runs in vacuum or inert gas or in air above about 500 °C, use boron nitride. Mixed films exist, but they usually inherit graphite’s temperature limit and colour, so they rarely make sense outside a few niche duties.
Common Questions
Is boron nitride better than graphite?
Neither is better; they suit different jobs. Boron nitride wins on temperature in air, cleanliness, electrical insulation and non-wetting to aluminium and glass. Graphite wins on cost and is the standard for hot forging of steel.
Can I mix boron nitride and graphite?
Physically yes, but the blend oxidises at graphite’s limit and leaves a grey film, so you pay for BN without getting its two main benefits. Choose one for the duty.
Why is boron nitride more expensive?
It is a synthetic ceramic made at high temperature from boron and nitrogen compounds, whereas graphite is mined or made from carbon at lower cost. The price gap narrows once consumption per part and the cost of rejects are counted.
Is hexagonal boron nitride the same as cubic boron nitride?
No. Same compound, different crystal structure. Hexagonal BN is soft and lubricating; cubic BN is nearly as hard as diamond and used for cutting tools. See the hexagonal vs cubic guide.
What temperature can each take in air?
Graphite begins to oxidise at roughly 450–500 °C; hexagonal boron nitride at roughly 850–900 °C. In vacuum or inert gas both go higher, but graphite loses lubricity in vacuum while BN keeps it.
Tell Us The Process; We Will Recommend The Grade
Substrate, process temperature, atmosphere and how you prefer to apply it. That is everything we need to point you at the right product, send a sample, or quote.
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